Most of us are aware of the high levels of mercury found in fish. But where does this mercury come from?

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Humans have been using mercury since before the Industrial Revolution, but it is currently being emitted by coal-fired power plants and artisanal gold mining. And according to researchers at Harvard University, significant reductions in mercury emissions will be necessary because of the element’s persistence in surface reservoirs from past pollution.

Mercury particles then end up getting released into the air which end up raining down into water bodies and absorbed into the soil. Eventually these microbes in aquatic ecosystems convert mercury into methylmercury which accumulated in fish and consequently can cause health effects for those who consume these fish products.

"It's easier said than done, but we're advocating for aggressive reductions, and sooner rather than later," says Helen Amos, a Ph.D. candidate in Earth and Planetary Sciences at the Harvard Graduate School of Arts and Sciences and lead author of the study, published in the journal Global Biogeochemical Cycles.

Their model reveals that most of the mercury emitted to the environment ends up in the ocean within a few decades and remains there for centuries to millennia.

While mercury is naturally released during volcanic eruptions, humans are the main culprit to the pollution.

"Ideally, mercury released by human activities would quickly be sequestered in the environment, but instead what we see is a huge quantity of it bouncing from one reservoir to the next," explains senior author Elsie M. Sunderland, who is the Mark & Catherine Winkler Assistant Professor of Aquatic Science at the Harvard School of Public Health. "This means it continues cycling throughout the environment and persists for much longer timescales than most people realize, which has implications for long-term biological exposures."

"Today, more than half of mercury emissions come from Asia, but historically the U.S. and Europe were major emitters," says second senior author Daniel J. Jacob, Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering at Harvard SEAS. "We find that half of mercury pollution in the present surface ocean comes from emissions prior to 1950, and as a result the contribution from the U.S. and Europe is comparable to that from Asia."

Sunderland notes: "Our study reinforces the need for immediate and stringent emissions controls globally, to the extent technologically possible, to avoid future human health risks. Already, the costs of methylmercury exposure in Europe and the United States have been estimated at upwards of $15 billion."